Many modern consumer devices require comfortable user data entry and proximity sensing interfaces which became very popular in devices like smart phones and portable media players. The measurement principles applied in position sensing are many-fold. The following measurement principles are commonly applied: Capacitive Proximity Position Sensors, Resistive Position Sensors, Optical Position Sensors and Acoustical Position Sensors.
The first two principles are the most popular ones and cover in total more than 90% of all position sensing applications. The measurement of touch dependent resistances and proximity dependent capacitances is utilized to obtain the position information by numerical post-processing. An integrator circuit is used to transform a resistance or a capacitance into timing information that can be captured by a microcontroller unit (MCU). The integrator is stimulated by an input signal and the resulting response is sampled and hold and evaluated by an MCU. Another common approach is to use a constant current to charge a capacitor under test and measure the time required to charge the capacitor to a predefined voltage. After the measurement, the capacitor is reset by a reset signal and a new charging cycle can be started. Another common method for capacitance measurement is to use a capacitor under test as timing element in a relaxation oscillator, resulting in a capacitance to frequency conversion. The resulting frequency is measured by a frequency measurement routine executed on a MCU.
A common capacitance measurement method is to use the capacitor under test as a frequency dependent resistor charging an integration capacitor in a switched capacitor integrator configuration. The basic principle is well known and documented, e.g., in the publications: Switched-Capacitor Circuit Design, R. Gregorian, et al, Proceedings of the IEEE, Vol 71, No. 8, August 1983 and Switched-Capacitor Circuit with Large Time Constant, Krishnaswamy Nagaraj, U.S. Pat. No. 4,894,620, Jan. 16, 1990.
When operating a prior art capacitive proximity sensor in a contactless smartcard the sensitivity of the sensor degrades significantly due to the extremely poor coupling between real earth ground potential and the smartcard's circuit ground potential. The poor ground connection results in a reduced sensitivity that does not allow implementing a 2-dimensional sensors for online-handwriting recognition systems in a contactless smartcard system.
When operating a prior art proximity sensor based on a plurality of individual sensors utilized to obtain a position by applying a center of gravity algorithm on the individual sensor's activity levels so called ghost positions are identified that appear due to a combination of random noise and the nature of the center of gravity algorithm. These ghost positions reduce the position quality such that it may cause false signal processing in i.e. a handwritten character recognition algorithm.
When operating a prior art proximity sensor based on a plurality of individual sensors in a contactless smartcard the poor sensor sensitivity may be improved by extending the integration interval. Prior art proposes to increase the integration capacitor. Increasing the integration capacitor extends the measurement time and consequently reduces the amount of measurements that can be executed within one second. The amount of measurements that can be achieved within one second according to the prior art proposal is too small for a proper detection of handwritten characters.
When operating a prior art proximity sensor based on a plurality of individual sensors only one sensor is measured at a time which reduces the total effective sensor area to the size of an individual sensor. Noise picked up by that sensor has a relative large impact on the sensor signal.